Antenna arrays are used in applications such as radars and beam-based communication systems. For example, see R. Mailloux, “Phased Array Antenna Handbook,” 2nd edition, Artech House, 2005; D. Parker and D. Zimmermann, “Phased Arrays-Part I: Theory and Architectures,” IEEE Trans. Microwave Theory and Techniques, vol. 50, March 2002; D. Parker and D. Zimmermann, “Phased Arrays-Part II: Implementations, Applications, and Future Trends,” IEEE Trans. Microwave Theory and Techniques, vol. 50, March 2002. The main reason for using antenna arrays is their capability of generating special dynamic radiation patterns such as steerable beams without any mechanical movement.
In general, each array element of an antenna array consists of a passive antenna called an antenna element or simply an antenna and a circuit block. Usually, the antenna elements are placed on a regular grid. The pitch of this grid is approximately or equal to half the wavelength at which the antenna array operates. The circuit block of the array element may be as simple as a passive phased shifter or as complex as an entire radio including amplifiers, mixers, filters, data converters, and digital circuits. If the antenna array contains only passive components, it is called a passive antenna array. If the antenna array contains active components, it is called an active antenna array.
Antenna arrays can generate many radiation patterns. For example, they may accept incoming signals from certain directions and block incoming signals from other directions or may transmit only narrow beams even though each antenna element radiates widely. Such radiation patterns are extremely useful in many applications. In radar using antenna arrays, the transmitted signals are focused in a particular spatial direction and the only accepted signal reflections are from the same direction without any physical movement of antennas as in conventional systems. In wireless communication systems, using spatial channels for transmitted and received signals, a technique usually called beam steering, increases the communication system capacity substantially.
The way an antenna array generates special dynamic radiation patterns is by properly combining the received signals from the antenna elements in receive mode and by properly exciting the antenna elements in transmit mode. Depending on how these operations are performed, antenna arrays are called either analog or digital. In analog arrays the receive and transmit radiation patterns are formed by analog circuits while in digital array they are formed by digital processing under software control.
To date, the most successful analog antenna arrays are the traditional phased arrays. Historically, the Passive Electronically Steered Array (PESA) has been developed first. This design uses a signal distribution/combining network called a corporate feed and passive adjustable phase shifters at each antenna element. These components have high loss limiting the system performance. Adding Receive/Transmit (Rx/Tx) amplifier modules per individual antenna element helps this problem resulting in Active Electronically Steered Array or AESA, currently the prevalent military radar architecture. For both PESA and EISA, the corporate feed and the programmable phase shifters are high performance expensive components.
The corporate feed is a passive tree network made of multiple transmission line sections interconnected with multiple splitters/combiners. The corporate feed has one input/output (I/O) port connected to the beginning of the tree trunk and many I/O ports connected to the end of the top branches of the tree. The network is electrically symmetric such that a signal applied at the trunk port arrives simultaneously at all branch ports. The network is reciprocal such that signals applied at the branch ports travel the same amount of time to arrive at the trunk port. In other words, the flight time of the signal from the trunk port to any branch port and vice versa is a constant. In addition, the corporate feed is a signal-combining network. When different input signals are applied to the branch ports simultaneously, the signal at the trunk port is the sum of these input signals. The practical realization of the corporate feed is expensive because this network contains many signal splitting/combining operations and because the transmission line sections must be accurately matched in length and terminated electrically with accurate impedances. All these design conditions are error prone.
While at one end of the antenna-array technology spectrum are the traditional PESA/EISA phased arrays, which generate radiation patterns exclusively with analog methods, at the other end of this spectrum are software-configured digital systems. Typically, these systems use 4-12 independent radios connected to 4-12 independent antennas, respectively. There are no physical connections between these radios or these antennas. Each radio contains data converters converting the received signals from analog format to digital format and converting transmitted signals from digital format to analog format. The respective 4-12 digital transmit and 4-12 digital receive signals are generated and/or processed by a digital signal processor under the control of special software usually called “beam forming/steering” software.
The software-configured digital arrays can be readily built with standard hardware and are extremely flexible in terms of programmability but suffer from fundamental shortcomings. First, the hardware of these systems is naturally expensive since there are many (4-12) radio systems present. Furthermore, these radios must have very high performance to ensure that the digital representations of the antenna signals (which are always analog) are correct. Second, the software generating the signals is extensive and runs in real time, requiring substantial processing power. Third, having only 12 or less antennas per system limits the array performance. A common compromise is to form dynamic patterns (e.g. beams, etc.) only in azimuth (horizontal directions) with fixed elevation (vertical direction) patterns. In the case of forming beams, typically, these are elongated cones spanning narrow but long regions. In contrast to this, a PESA/EISA analog phased array with hundreds or thousands of antennas generates narrow round beams steerable in both azimuth and elevation.
In principle, the number of antennas in software-configured digital arrays can be scaled with a corresponding increase in system cost and size. A common approach to limiting the physical size of the system is to place as much of the radio hardware as possible on phased array panels. Such highly compacted digital arrays with many tens or even hundreds of elements are intended for applications, where cost is not a primary technology driver, such as some military radars.
Between the two technology extremes defined by all analog processing or all digital processing, there are other known possibilities for implementing active antenna arrays, partially with analog techniques and partially with digital techniques. For example, a large array may be segmented into many sub-arrays, each sub-array being designed as an analog system. However, the signals to/from each sub-array would be generated in the digital domain.